The language of birds: 1. Song

The word "song" implies a certain musicality or tunefulness, and was first applied to the regular springtime outpourings of birds because so many of them are tuneful. In bird biology, however, the word has come to acquire its own wider meanings: any sound - melodic or unmelodic - delivered in the nesting period, usually by a male bird, usually for the purpose of territorial defence or the attraction of a mate. Thus the harsh churring of the night jar Caprimulgus europaeus, the instrumental bleating of the snipe Gallinago gallinago and the wing-sound proclamation of the ruffed grouse Bonasa umbellus, while none of them is tuneful, are all "songs" in the biological sense.

The more typical songbirds belong to the sub-order called the Oscines, or true songbirds. They comprise almost half the birds that exist - over 4,000 species - and include, for example, such families as the thrushes (Turdidae), larks (Alaudidae), wrens (Troglodytidae), mockingbirds (Mimidae), the Old (Sylviidae) and New World (Parulidae) warblers and so on.

The purpose of song is self-advertisement, whether it is from a mockingbird Mimus polyglottos amid the magnolia flowers, a European robin Erithacus rubecula among the apple blossom, a tawny owl Strix aluco under the moon or from a green-legged tree partridge Arborophila charltoni delivering its surprisingly tuneful message from the depths of a south-east Asian rain forest. In some species the function is solely to declare ownership of a territory and in others solely to attract a mate; but in many it is to achieve both. Great tits pair up before the male starts to sing and thus under normal circumstances his song is exclusively territorial, but should a male suddenly lose his mate; he would then sing at six times his normal rate until she was replaced. Sedge warbler Acrocephalus schoenobaenus cocks, having overwintered south of the Sahara, arrive back in springtime Europe a few days ahead of the females and immediately stake out territories, these they defend by chasing intruders out. Song which occupies much of their time (by both day and night since females might arrive at night) ceases abruptly when the cock acquires a mate. The European robin's spring song has a dual function; the one signal conveying a different meaning depending upon the sex of the listening robin: an intruding male will be repelled, a passing female attracted.

Property and sex are clearly important, and this importance is reflected in the tremendous amount of effort put into singing by the cock birds. A male yellowhammer Emberiza citrinella may chant "a little bit of bread and no cheese" 3,000 times a day, half a million times in a season, and according to the physiologists, singing takes almost as much energy as flying. If, through powerful binoculars, you watch a yellowhammer delivering a song, you can see how he puts everything into it, his whole body quivering with the exertion.

Besides the frequency of reiteration, the carrying power is also
of significance in getting the message across as effectively as
possible. So too will be the choice of perch, many species picking
really prominent positions from which to issue their proclamation.
The mistle thrush Turdus viscivorus, for example, will
broadcast its strident song from the pinnacle of a tree, so as to
cast as broadly as possible its message. The choice of singing position
will obviously be limited by the opportunities in the bird's preferred
habitat. Skylarks Alauda arvensis live in open country
and therefore have to sing in flight if they are to gain elevation.
Meadow pipits Anthus pratensis and rock pipits A. spinoletta
also lack suitable natural song posts and have taken their self-advertisement
one stage further, they perform a choreographed song flight, adding
a distinctive visual element to the show - a TV commercial, not
merely a radio one! So too does the tree pipit A. trivialis.

Birds that are active only at night have to rely heavily on sound to give notice of their presence, hence the well-developed and characteristic song of the owls (Strigidae), the night jars (Caprimulgidae), and of other less well-known nocturnal families like the tropical potoos (Nyctibiidae).

Even diurnal birds that live in thick undergrowth or deep woodlands are very dependent on sound. Indeed, one of the great advantages of sound signalling as distinct from visual signalling is that sound does not demand "line of sight" for its transmissions; it will go round corners. To take one among scores of examples, the Australian Lyrebird Menura novaehollandiae lives in such thick vegetation that males cannot see one another if more than two or three metres apart, and yet their song (admittedly very loud and continuous) keeps them spaced out and up to a 1,000 metres apart. Certain kinds of sound are less "blanketed" by vegetation than others, and so we find that the songs of birds that live in dense habitats have been adapted to "get through" more easily. For instance, great tits that live and sing in thick forests produce lower pitched songs than those that live in more open woodlands. It has even been suggested that in Scandinavia the urban great tits have changed to fight traffic noise. An incisive two-note song having largely replaced a three-note song commoner in the days of fewer cars.

It will be clear that members of each species communicate only with other members of their own species: there is not one "song language of birds" but as many as there are species of birds - thousands! Indeed, one of the more elementary functions of song is to enable females to recognise males as being of the "right" species. Humans, too, distinguish unseen birds by their species- specific songs.

Gilbert White, the gentle curate of Selborne and the father of English natural history, was the first to sort out from one another three very similar-looking birds, by their songs, the willow warbler Phylloscopus trochilus, wood warbler Ph. sibilatrix and chiffchaff Ph. collybita. That was 250 years ago. Today in tropical forests virtually the only way to know which birds are about, is to know their songs and calls.

Willow Warbler Phylloscopus trochilus

Chiffchaff Phylloscopus collybita

Wood Warbler Phylloscopus sibilatrix

Sonagrams to show the very different songs of three similar-looking
bird species

However, not only is the song specific to each species, in some species at least (and perhaps in many) each individual male singer can be identified by other males by his song. Make a recording, convert it via a sonagraph into a "voice print" and one has, in effect, a finger print, a sound signature. Not only can we tell individual cocks apart in this way - so can the cocks themselves by listening to one another.

This was first proved in the case of a New World warbler called the ovenbird Seiurus aurocapillus. Imagine bird A settled in its territory, with three neighbours 1, 2 and 3. All four are singing normally and none worries unduly about his neighbours. If, however, you introduce the taped voice of another bird, B, (previously recorded from a territory several miles away) playing it back within earshot of A, then A instantly recognises the "bird" as a stranger and at once investigates. Individual signature songs and the ability of ovenbirds to recognise them, functions to save territory holders time and energy. Once identified there is no need to investigate birds who are "next door", because experience has shown how they usually operate.

This is not to suggest that neighbours are necessarily well behaved.
Bird 1 could well set out to annexe part of A's territory, but A
would presumably notice him visually or detect that his song sounded
clearer. Thus it is important to know that your neighbour is keeping
- or not keeping! - his distance. The loudness of his song will
give a crude measure of his distance but to help farther territory
holders of certain species to monitor the behaviour of neighbours,
a special technique has evolved.

Certain species like the chaffinch and the great tit do not sing exactly the same phrase each time they sing, each individual has a repertoire of several phrases, many of which it shares with its neighbours. If you are in the presence of either of these species it soon becomes noticeable that each individual which initiates a bout of singing is almost invariably answered with the phrase most closely resembling the one that he himself used. The counter singer, in other words, matches the song he hears -goes "snap!". Why? Well, the suggestion is that by this means the answerer (and for that matter the initiator) can best judge the distance that separates the two. Each knows what that precise phrase sounds like to himself as he sings it, and can therefore, if the same phrase is used back, make the best possible comparison, determining the degree of degradation in the signal and thus the distance of the signaller. It may sound a little far-fetched, but it is certain that song-matching takes place, and this is one plausible explanation.